Cardiac rhythm is maintained by precisely timed nerve signals electrically exciting and being conducted through cardiac tissue to stimulate synchronous contractions of the four heart chambers (2 ventricles and 2 atria). In a normal sinus rhythm, the nerve signals are typically conducted along paths initiating at the sino-atrial (SA) node and passing from there through the atrioventricular (AV) node and the bundle of His to the ventricular myocardial tissue.
Potentially dangerous abnormal cardiac rhythms, or arrhythmias, including atrial fibrillation, are common medical conditions which may result from disturbances in the site of origin and/or the pathways of conduction of the nerve signals exciting contraction of the four chambers of the heart. The site of origin and pathways of conduction of these signals are currently mapped, for example using an electrocardiograph (ECG) in conjunction with mapping methods such as those described in U.S. Pat. No. 4,641,649 to Walinsky et al.
One common type of atrial fibrillation occurs when the contraction initiating signals originate within one or more of the pulmonary veins rather than at the SA node. These atrial arrhythmias have been treated by a variety of methods including pharmocologic treatments, highly invasive surgical procedures and linear and circumferential RF ablations of the myocardial wall. However, each of these methods has drawbacks, e.g., the pain and extended recovery time for invasive surgery, relative ineffectiveness of pharmacologic treatments and restenosis at the ablation site due to the application of RF energy or other heat based therapies thereto.